1. Field of the Invention
The present invention relates to a pneumatic tire, more particularly to a metal cord having a specific initial elongation being capable of improving the tire durability.
2. Description of the Background Art
In the pneumatic tires, steel cords are widely used as tension members to reinforce various tire components, for examples, a carcass in a heavy duty tire, a tread reinforcing belt, a bead reinforcing layer and the like.
In the case of the carcass of the truck/bus tires for example, “1×n×d” cord structures and layered cord structures “2×d+6×d”, “3×d+8×d”, “3×d+9×d” and “3×d+9 ×d+1×d” are usually used in the ply. AS well known in the tire cord art and as described in the Japanese Industrial standard G3510 “Testing methods for steel tire cords”, the “1×n×d” cord structure indicates that a number (n) of steel filaments having a diameter (d) are, as one bunch, twisted together into a cord. A layered cord structure has a core, a sheath and optionally a wrapping wire. For example, “3×d+9×d” indicates that a core is made up of three steel filaments having a diameter (d) and an outer sheath is made up of nine steel filaments having the diameter (d). “3×d+9×d+1×d” indicates that a steel filament having the diameter (d) is wound around the “3×d+9×d” structure as an additional wrapping wire.
In recent years, on the other hand, in order to improve the penetration of the topping rubber into such cord structures, the use of waved filaments has been proposed.
When the waved filaments are twisted, gaps are formed between the filaments, and the penetration of topping rubber into the cord is facilitated.
Getting another perspective on the formation of the gaps, as the filaments are not firmly contact each other, when the contacting condition is varied, cord characteristics, e.g. elongation, tensile modulus and the like are also varied. This is especially remarkable when the cord load is light.
When a cord just manufactured is measured, the variations of the cord characteristics are probably small as the cord is long and a suitable tension is applied thereto. But, when the cords are cut into a certain length are laid parallel with each other in a form of rubberized ply or layer, the variations are increased.
Therefore, when the cords are used to reinforce the carcass of the heavy duty radial tires, as the carcass cord tension is very large in comparison with the other usages such as tread reinforcing belt and bead reinforcing layer, the variations of the cord characteristics have an adverse affect on tire performance.
More or less, the carcass cords embedded in the tire grow or their lengths increase during use. In particular, when the initial elongation of the cord is larger, the cord becomes longer. Thus, if the variation of the initial elongations of the cords is large, the tire uniformity is deteriorated. Further, due to the different cord lengths, the load concentrates on the shorter cords. Thus, the shorter cords are liable to fatigue more than the longer cords, and the durability is decreased.